In a typical diaphragm valve of the prior art a valve body includes inlet and outlet passages communicating with a valve chamber with a seating ring surrounding the region where one of the passages connects with the chamber. A flexible diaphragm is mounted in the valve body and is selectively moved away from the seating ring to permit fluid flow through the valve or pressed against the seating ring to block fluid flow. A handle is rotated to move a threaded valve stem which engages a threaded valve bonnet so that longitudinal movement of the valve stem forces the diaphragm to seat against the valve seating ring.
If, after the diaphragm has seated against the seating ring, the handle is rotated to move the valve stem further in the direction of the diaphragm, an excessive force may be exerted against the diaphragm causing it to rupture or fail. Using a thicker diaphragm will not solve the problem because a thick diaphragm is less flexible than a thin one and, because it is less flexible it has a greater tendency to crack as a result of the force repeatedly exerted against it.
In some valves of the prior art, the problem of diaphragm failure by rupture or cracking is overcome by providing a stop ring for limiting movement of the valve stem in the direction of the diaphragm. However, if the stop ring also serves as a clamp for holding the peripheral edge of the diaphragm, a further problem is introduced. After the diaphragm has seated against the seating ring further rotation of the handle causes a rotational force to be exerted via the stem and the stop ring to the diaphragm causing a rotational movement of the diaphragm with an attendant loss of air-tightness of the valve chamber. The prior art solves this problem by employing a diaphragm comprising a plurality of generally flat thin plates which may slide relative to each other so that rotational movement of the stop ring is not transmitted to the plate which seals the valve chamber.
It is difficult to handle the flat thin plates and during assembly they are frequently dropped and/or damaged. They cannot be assembled as a unit hence they must be placed in a valve one plate at a time. Furthermore, it is difficult to remove the flat plates from a valve, particularly in instances where it is desirable or necessary that the valve be repaired without removing it from the pipe or conduit in which it is installed.